Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with none zero spin have a magnetic moment. In medical imaging, usually nuclear hydrogen atoms are studied since they are present in the body in high concentrations for example water. The nuclear spin of elementary particles can resonate at a resonance frequency, if a strong DC magnetic field (B0 field) is applied. The magnetic resonance (MR) frequency is determined by the level of the magnetic flux. In an MRI scanner, the magnetic field matches a selected resonance frequency only at specific positions in space. By varying these resonant positions step by step, the measured MR signals can be reconstructed to an image.
The needed strong DC magnetic field is typically generated by superconducting magnets. In order to vary these fields, such that it matches a given radio frequency only at one position, a field gradient is generated using gradient coils. Thereby, the field gradient can vary over time to achieve a scan. The frequency range in the gradient coils is low and reaches up to a maximum of 10 kHz.
Shielded gradient coils for cylindrical MRI systems mostly consist of a cylindrical inner coil and a cylindrical shielding coil at a bigger radius. x, y, and z coils are typically arranged in separate layers. Further, an RF (radio frequency) body coil is typically located at the inside of the gradient coil with a certain radial thickness to allow for effective RF field generation. The net space available for an object to be scanned in the MRI scanner, for example the patient, is thus determined by the inner radius of the RF coil, including covers. Increasing the inner radius of the gradient coil drastically increases the required input power per gradient field, and therefore cost of the gradient amplifier driving it. Thus, a tradeoff has to be made between cost and available patient space.
For example, WO2008/053451 A1 does disclose an MR system comprising a gradient coil that does not comprise patient comfort. The gradient coil comprises a first coil portion and a second coil portion located at different distances from the central axis in a bore-type system.
It is a goal of the invention to provide an improved MR system comprising an improved gradient coil.